The present invention relates to equipment for wrapping groups of products in plastic film, and particularly in heat-shrinkable film.
Such equipment is intended for use in packaging machines, and in particular, machines designed to handle items of cylindrical or prismatic geometry such as bottles, cans or the like.
Automatic machines of this type reflecting the current state of the art present a processing and infeed line on which bottles (the term xe2x80x9cbottlexe2x80x9d is used in the interests of simplicity, and does not imply the exclusion of other products exhibiting different shapes) are advanced through successive intermediate stations, where the selfsame bottles or other products are positioned and ordered into groups, and stations where the groups are enveloped or packaged in wrappings of plastic material.
The function of the intermediate positioning and ordering stations is to gather the single products into groups of predetermined number, which can vary according to the selected size and style of pack (in the case of bottles, for instance, the single group could consist in two rows of three bottles) and then to direct the selfsame products toward the aforementioned wrapping stations. Thereafter, the group leaves the wrapping station and passes along a conveying line to a final station at which the pack is completed, normally by applying a layer of heat-shrinkable film.
The wrapping station in question is equipped with a feed system by which the heat-shrinkable plastic film (used to form the wrappings) is decoiled from a roll and supplied continuously from a relative station located below the level of the conveying surface occupied by the products.
The film is supplied from the roll to a unit by which it is measured and cut into sheets of length sufficient to loop over the groups of products in a direction coinciding with the conveying direction and reach down to the conveying surface occupied by the selfsame products.
The conveying surface is surmounted by a special wrapping cage of which the function is to take up the film (onto cross rails carried between chains extending parallel on either side), as it emerges from a slot, and draw it over the groups in such a way that they can be wrapped during their movement along the conveying surface.
In practice, the film decoils along the conveying direction followed by the products and is wrapped around the group of products in such a manner that when the operation is completed, the ends of the wrapping sheet are overlapped whilst the sides of the group coinciding with the two vertical lateral faces of the pack are left exposed. The wrapping is drawn taut over the group of products by exploiting the heat-shrink properties of the plastic film, in a manner familiar to persons skilled in the art.
Attempts have been made, with machines having this type of structure, both to improve the output per unit of time and to reduce unproductive down times, for example as when changing from one size of pack to another: the changeover involves various operations affecting the drive systems (for example that of the decoiler and cutting station, to alter the length of the single sheet of film), and a number of mechanical adjustments, such as altering the height of the structure that carries the cross rails serving to lift and guide the film, effected by means of hand-operated linkages.
In this context the applicant has designed and implemented a solution for a feed and cut unit (disclosed in EP 839 723), positioned immediately below the slot through which the decoiling film is fed to the wrapping surface and comprising a pair of contrarotating rollers between which the film is interposed and gripped tangentially. A first of the two rollers is equipped with a radially projecting knife, whilst the second affords a radial groove positioned to accommodate the knife when a cut is made during each rotation, and presents a flat on the cylindrical surface designed to create a gap through which the film is able to pass freely.
The structure in question significantly improves the feeding and cutting action, rendering it almost continuous, and enables a swift size changeover as regards the length and preparation of the portion of film needed to wrap the group of products.
Nonetheless, there are still substantial problems with adaptation of the wrapping unit to a different size of pack, since the cage structure needs to be adjusted from both sides (by means of handwheels), altering the number and/or position of the cross rails by which the film is lifted and guided.
The object of the present invention, accordingly, is to overcome the drawbacks mentioned above by embodying equipment for wrapping groups of products in plastic film in such a way as to speed up and synchronize the operations of feeding and cutting the film, and rationalize size change operations insofar as they affect the unit by which the groups of products are wrapped, through the adoption of a simple, precise and compact structure.
The stated object is realized according to the present invention in equipment for wrapping groups of products in plastic film, comprising a first surface along which products are conveyed toward a second surface on which the selfsame products are formed into groups and wrapped each in a sheet of film by relative wrapping means, also a feed unit, by which the film is advanced and cut into sheets of length determined according to the dimensions of the group of products being wrapped, positioned beneath and in close proximity to a slot separating the first surface from the second surface.
Advantageously, the wrapping means comprise at least one cross rail carrying and guiding the sheet of film, supported at one end by a single power driven arm located in close proximity to one side of the second surface, and adjustment means acting at least on the power driven arm, such as will enable the cross rail to rotate through a first active trajectory passing above the second surface and a second return trajectory passing below the second surface, describing a curved path of which the distance from the second surface is variable and selectable according to the dimensions of the groups of products being wrapped.